Such servo valves are used in a number of different forms. Representative uses for such valves are to be seen in compressed air and hydraulic circuits for motor vehicles, more specially trucks. For such purposes the servo valves have to be produced in generally small production runs with different design details, as for example for working pressures of 10 or 16 bar. One shortcoming of known servo valves is that it is hard to make small production runs of customized valves at low prices. A further point is that, if for some reason or other the holes for fixing the valves in place and the screws or the like for this purpose are to be placed nearer to each other, there will be the further shortcoming that the valves have to be made smaller in size, this cutting down the rate of flow therethrough, that is to say the size of the ducts for the controlled fluid.